1. Field of the Invention
The present invention relates to a failure detection system for detecting failures of an air-fuel ratio control system for feedback controlling an air-fuel ratio of an internal combustion engine.
2. Description of Related Art
In an air-fuel ratio control system, in which an air-fuel ratio is regulated, for instance, by controlling an injected quantity of fuel, a quantity of fuel to be injected, which is obtained as a basic value of control based on the quantity of intake air introduced into an engine and a rotational speed of the engine, is corrected in accordance with the temperature of engine cooling water, etc. In addition, the basic amount of fuel injected is feedback controlled in accordance with the oxygen (O.sub.2) content of exhaust gas as an actual fuel injection amount so as to regulate an air-fuel ratio to a target air-fuel ratio. Since attempts are made in such an air-fuel ratio feedback control system to maintain a stoichiometric air-fuel mixture for providing a theoretically ideal combustible air-to-fuel ratio, the control system allows a three-way catalytic converter to purify exhaust gas effectively.
In order to compensate both for variation in the output properties and for degradation due to time of an air-to-fuel ratio sensor of the air-fuel ratio feedback control system, it has been proposed to dispose another air-to-fuel ratio sensor downstream of the catalytic converter. In such an air-fuel ratio feedback control system, when the upstream air-to-fuel ratio sensor outputs a signal indicating that an air-to-fuel ratio has changed from a rich side to a lean side or vice versa, a set time of delay processing is corrected in accordance with an output from the downstream air-to-fuel ratio sensor. Further, in order for the air-fuel ratio feedback control system, having two air-to-fuel ratio sensors disposed upstream and downstream of a catalytic converter, to minimize a drop in responsiveness due to functional degradation of the upstream air-to-fuel ratio sensor, a skip value (P-value) necessary to perform air-to-fuel ratio control by the use of the upstream air-to-fuel ratio sensor is corrected in feedback control by means of an output from the downstream air-to-fuel ratio sensor. This control is called "P-value feedback control" in the automobile art and is known from, for instance, Japanese Unexamined Patent Publication No. 61-234,241. Since variation in output properties of the downstream air-to-fuel ratio sensor reflects functional degradation of the upstream air-to-fuel ratio sensor, functional failures due to degradation of the upstream air-to-fuel ratio sensor can be detected from an output from the downstream air-to-fuel ratio sensor.
Typically, in the P-value feedback control, while the downstream air-to-fuel ratio sensor outputs a signal indicating that the ratio remains relatively rich, a skip value for changing an air-to-fuel ratio from a rich side toward a lean side is feedback controlled and decremented at a rate of, for instance, 0.2% every specific time period of, for instance, 8.2 milliseconds. Simultaneously, a skip value P for changing an air-to-fuel ratio from a lean side toward a rich side is incremented at the same rate during the same specific time period. Conversely, while the downstream air-to-fuel ratio sensor outputs a signal indicating that the ratio remains on a relatively lean side, a skip value P for changing an air-to-fuel ratio from a rich side toward a lean side is changed by the same increment rate during the same specific time period. Simultaneously, a skip value P for changing an air-to-fuel ratio from a lean side toward a rich side is changed by the same decrement rate during the same specific time period.
Functional degradation of an upstream air-to-fuel ratio sensor, such as an oxygen (O.sub.2) sensor, is classified into three forms. These forms include a lean shift degradation, which shifts the center of control of an air-to-fuel ratio toward a lean side; a rich shift degradation, which shifts the center of control of an air-to-fuel ratio toward a rich side, and a frequency degradation, which causes a drop in responsiveness so as to make the frequency of an output signal wave lower. Both the lean shift degradation and the rich shift degradation are detected based on whether a skip value P for air-to-fuel ratio feedback control is above or below a critical level of skip value P or a threshold skip value. On the other hand, the frequency degradation can be known from an increase in ratios of frequencies of output signal waves from the upstream and downstream air-to-fuel ratio sensors.
The P-value feedback control is also utilized to detect functional degradation or a purification efficiency of a catalytic converter between the upstream and downstream air-to-fuel ratio sensors from a ratio of frequencies of output signal waves from the upstream and downstream air-to-fuel ratio sensors. If the functional degradation is above a critical level, the catalytic converter is considered to have failed.
A feedback correction value of a skip value depends upon the time for which an output from a downstream air-to-fuel ratio sensor remains on a rich side or on a lean side, which depends upon the functional performance of a catalytic converter. Consequently, according to functional degradation of the catalytic converter, the remaining time becomes too long to keep a skip value within proper limits or, otherwise, becomes to short to converge to a skip value. If the detection of lean shift degradation and rich shift increase of the upstream air-to-fuel ratio sensor, which is used to perform the air-to-fuel feedback control, is tried by comparing a skip value, corrected by the P-value feedback control, with a threshold skip value, a failure in detection can be caused. This results, on one hand, from overshooting of the skip value due to a very active catalyst and, on the other hand, from a non-converged skip value, due to a great deal of degradation. That is, if the catalytic converter has been less degraded and is at a high exhaust gas purification efficiency, it keeps a high oxygen storage effect. Consequently, an output from the downstream air-to-fuel ratio sensor remains on a rich side or on a lean side for a long time, so as to excessively increase a skip value. This leads to a proper detection of functional degradation of the upstream air-to-fuel ratio sensor. On the other hand, if the catalytic converter has been greatly degraded and is at a low exhaust gas purification efficiency, it suffers a drop in oxygen storage effect. Consequently, an output from the downstream air-to-fuel ratio sensor changes on a short cycle, so as to disable a skip value from converging with the threshold skip value. This also leads to a proper detection of degradation of the upstream air-to-fuel ratio sensor.
In the system for monitoring degradation of the upstream air-to-fuel ratio sensor based on an output from the downstream air-to-fuel ratio sensor, since it is hard for the downstream air-to-fuel ratio sensor to raise the temperature of the catalyst to its active temperature at the beginning of operation, it is generally heated for early activation by a heater. However, heating the air-to-fuel ratio sensor accelerates thermal degradation of the air-to-fuel ratio sensor while an engine operates at high speeds and under high loads. This results in a failure of detection.
When monitoring a ratio of frequencies of output signal waves from the air-to-fuel ratio sensors disposed upstream and downstream of the catalytic converter, respectively, so as to detect a frequency degradation of the upstream air-to-fuel ratio sensor, if the air-to-fuel ratio sensors become so poor in responsiveness that a long time is needed to make a decision as to whether the air-to-fuel ratio is lean or rich, an air-to-fuel ratio may possibly change beyond limits available to the catalytic converter, so that the catalytic converter loses its oxygen storage effect. As a result, the downstream air-to-fuel ratio sensor outputs a signal wave as if the catalytic converter has suffered functional degradation. Consequently, despite the fact that the air-to-fuel ratio sensor operates normally, it is judged to operate incorrectly if the catalytic converter has been degraded.